ts-P3 and ts-P1 mutations in ts-1A2 recombinant viruses specified a satisfactory level of attenuation of influenza A viruses in man. Suppressor and other mutations resulted in loss of ts phenotype of a ts-1A2 recombinant during viral replication in a completely susceptible child. The mutation that suppressed the ts-P3 gene was located on the P2 gene. A new "ts-1(E) like "donor" virus containing ts-P3 and ts-NP genes was produced and found to be attenuated and genetically stable in hamsters. Characterization of the ts phenotype of A/AA/6/60 ca recombinants suggested that attenuation of the ca recombinants in man is not dependent solely on the ts property. ELISA's were developed to quantitate IgG, IgA, and IgM influenza A or B virus hemagglutinin antibodies in serum and nasal wash specimens. Squirrel monkey model of influenza A virus infection was extended to permit repeated sampling of the lower respiratory tract for virus without injury to the animal.